I pay $50/month for electricity in my apartment. If I had no other options, I am pretty sure I'd be willing to pay something like $5000/month for that electricity. The economic surplus for electricity is enormous, the price comes no where near measuring the utility it provides.
What you're talking about is consumer surplus, which is the difference between the vslue of a product to you and the price you pay. It is not an externality, because you don't benefit unless you buy (at presumably a market price).
We're not talking about the value of electricity, we're talking about the value of "cheap electricity." Attempting to assign a value to cheap electricity beyond its price is just double counting. There is no value to "cheap electricity" that isn't accounted for in the price and therefore it's not a positive externality. Put another way, the value you get from one unit of electricity is the same no matter whether it comes from coal or wind power. Any difference between the two is merely the incremental benefit of getting it at a cheaper price, which is entirely accounted for by the price.
